<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>验证</title>
    <style type="text/css">
        * {
            padding: 0;
            margin: 0;
        }

        .wrap {
            background-color: yellow;
            width: 800px;
            margin: 100px auto 0;
            height: 300px;
            font-size: 30px;
        }

        .box {
            background-color: green;
            margin-left: -100px;
            height: 200px;
        }
    </style>
</head>
<body>
<div class="wrap">
    最外层的宽度为800px
    <div class="box">里层的元素设置了margin-left:-100px</div>
</div>
</body>
</html>